does internal door need being painted?

[Mazonas]

Hi,
is it necessary to get internal door painted? All of my internal doors are with a top side not being painted and have natural wood surface (as seen in a photo). This is how they have come from manufacturer as far as I understand. Does it need painting?
I had some problems with some of the door.. as they were not shutting properly.. and they have been sandpapered with the device. The person who sandpapered them suggested to paint sandpapered areas with the gloss paint as it would protect from humidity getting into the door .. otherwise the problem might re-occur. Is it true?

[dewaltdisney]

Yes, it is. Wood is hygroscopic which means it can absorb moisture and lose it during the natural humidity changes in a house through the seasons and living created moisture. Therefore it is best to seal wood all around. The doors are left with unfinished edges to allow for fitting hinges locks and trimming as necessary. This job does not have to be a lengthy process as water-based paints dry quickly. I use Johnsons water-based paint and their Joncryl primer undercoat followed by a top coat of gloss to match is easy enough to do in a day. You might not want to do the money so any paint will seal it, just prime first.

DWD